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Old October 4th, 2009, 06:31 PM   #1 (permalink)
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'60s Sears Silvertone 4707 organ amp conversion

Got a hold of an old organ amp. Before I started you could talk over it while dimed. Here's what I've done (so far)

I picked up one for 80.00. Rewired first stage and input like a typical blackface, 100k plate load and 1.5k cathode resistor bypassed with a 25/50v sprague, 1meg to ground with a 68k input resistor. I just used an rca to 1/4 adaptor. Tried bypassing the second stage but had waaaay too much gain. Left second stage stock. Replaced all bypass caps with .02 spragues, which took away the "boxy" sound it had, really openned up the sound. Replaced another 25/50v electrolytic with a sprague of the same value. Replaced the stock Oxford with a Weber alnico sig 12 speaker (45.00 wanted to go realatively cheap since it was a project).

Didn't have enough grind for me so I lowered the input resistor to 33k and raised the plate load resistor on the first stage to 200k, used 2 100k's in series. I would have rather lowered the last stage resistor on the filter cap to increase plate voltage but I didn't have the resistor I needed so I just raised plate load resistance. Plate load was around 165-170v before I raised the resistance (forgot to measure it afterwards).

I may experiment more with the second stage but, since an organ amp is pretty much a bass amp, I didn't want to loose too much of the low end. Gotta get some more components to play more with it.

I was trying to get what I wanted and still use the 12dw7, because the trem doesn't work on it and I wasn't sure if the extra oomph of a 12ax7 would affect the trem when I get to that. Stuck a 12ax7 there for more gain for now and I'm pretty much in Tweed territory, but I think with more minor tweeking (when I get the components I need) I may be able to do better.

Looks like something from the old "Dick Van Dyke" show from the early 60's. Very retro looking with the wooden legs. It's kinda like an end table looking tweed champ (vibrochamp when I get the trem working).

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Old October 4th, 2009, 07:41 PM   #2 (permalink)
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Sounds like fun stuff. What's in the output section?
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Old October 4th, 2009, 09:39 PM   #3 (permalink)
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It's a single-ended 6V6. Probably 5 or 6 watts. 12dw7 input stage that splits off the first 1/2 to the trem which is the first 1/2 of a 12ax7 the second 1/2 of the 12ax7 is the stage before the output. Basically it uses the 12dw7 as the input coupled to the second 1/2 of the 12ax7. The on switch is the trem knob, it has a separate volume. I once had an old Teisco ss amp that used the reverb knob as the on switch, weird huh?

Sounds good but still doesn't really grind til about 7 or 8. I'd like to get that down to about 3 or 4 on volume but I don't want to affect the trem circuit. That's my next thing, fixing the non-working trem.

Before I started, you could talk over it when it was dimed. Now it barks pretty well, just not soon enough for me.

Two of the caps I replaced with .02uf were 2200pf in series. This made a HUGE difference in its transformation. Not sure what thats going to do to the trem since the junction of the 2 goes to the trem circuit.
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Old October 4th, 2009, 10:30 PM   #4 (permalink)
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Hmm, with two 12AX7 sections feeding the 6V6, it ought to grind pretty early on the volume dial. My 5F2A starts breaking up around 3 when I have a 12AX7 installed (I prefer a 12AY7).

People get way too hung up over the idle plate voltage on their preamp stages. What's more important is to select the plate resistor for gain, the coupling capacitor for low-end rolloff, and the cathode resistor to bias it halfway between saturation and cutoff (unless you intentionally want the tube to clip early).

And yeah, if the amp doesn't have a power switch, it does seem weird to me to use anything but the volume knob!

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Old October 5th, 2009, 07:34 AM   #5 (permalink)
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I agree 100% about too much thought on plateload voltages. Starving them though gives you that buzzy preamp distortion. I'm still sorta playing with things. I should probably get the trem working first before I do anything else. That way I'll know if what I'm doing is going to affect it.

The second stage is biased for very low gain, very high, and unbypassed cathode resistor, around 4.7k If I remember right, and about 200k ohms on the plate load. The 4.7k is in parallel with a 1 meg. from the trem circuit. Don't have the hand-drawn schematic with me right now. Once again, this is if I remember right.

I'm an electronic printing systems specialist/tech so I'll see if our set-up guy can redraw it and make a nice print. I would really appreciate your input and would be better served with you taking a look at the scematic.

Thanks for the interest and input. Looking forward to more.
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Old August 22nd, 2010, 07:28 PM   #6 (permalink)
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My brother is looking at a Silvertone organ with this exact amp in it, I'm pretty sure. We couldn't get any sound out of the amp, though the reed section of it worked well and made sound. Did you get any experience with the amp when it was still in the organ? I'm trying to figure out what the heck this thing uses for a pickup...anyway, even if we can't sort that out, maybe worthwhile for the functioning reed air organ part and a potentially decent guitar amp project then?
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